Shadow into Sunlight — Elaine Shelabarger

In this, the second book about the Ashleigh family, Lucy -- now a young woman -- is heartbroken when her childhood sweetheart, James Wyatt, rejects her. To raise her spirits, her parents arrange for her to stay in Florence at the magnificent home of her great-grandmother, the Contessa di Lorenzo.
When she meets Major Charles Ryman, a dashing young American with a mysterious past, they are immediately attracted to each other. They meet in secret, flouting the Contessa's strict rules. And when Lucy must return to England, Charles follows her.
Lucy is deeply in love. But can she trust Charles' feelings for her? Is their love strong enough to survive the rumors about his past? And can Lucy give up everything to begin a new life with him in America?